The invention relates to the fields of engineering, microbiology, marine biology, physical chemistry, and fluid dynamics.
The invention provides for an outdoor large volume closed photobioreactor for the continued daily production of ethanol, or other biofuels, from a culture media comprising genetically enhanced cyanobacteria or algae and in situ separation of the ethanol from the culture media through evaporation by sunlight and subsequent condensation and ethanol collection in the photobioreactor. The photobioreactor apparatus is designed to allow for the maintenance of a high density, stable culture comprising genetically enhanced cyanobacteria or algae and separation and collection of the ethanol produced in the same apparatus. An embodiment of the invention is the removal of ethanol from the culture comprising genetically enhanced cyanobacteria or algae, wherein the ethanol is removed from the culture without additional external manmade energy.
Given the high and escalating cost of fossil fuel based transportation fuels, the enormous world-wide demand for such fuels and the negative environmental impact of the wide-spread use of these fuels, there has been a significant market driven shift to the use of alternative fuels that are cleaner and renewable, namely biofuels. Currently, the production of biofuels, particularly ethanol, is dominated by the conversion of high cost feed substrates such as sugar cane, corn, rapeseed, palm oil and other terrestrial crops predominantly used as food for human/animal consumption. While the technology exists to convert these feedstocks to ethanol and biodiesel for use as transportation fuels, there is not sufficient arable land or fresh water resources to meet the enormous demand of the global transportation fuels market. The United States alone uses over 140 billion gallons of gasoline for transportation fuel per year. The current U.S. output of ethanol made from corn is over 5 billion gallons annually. The economic impact of the diversion of significant amounts of corn from the human/animal food market to the transportation fuels market has caused a greater than 50% increase in the market price of corn on global commodity markets. Such impacts on food commodity markets are not sustainable in the long-term, and large amounts of effort are being expended to find renewable alternatives that are cheaper and have the potential for larger scale production.
The most predominant alternate technology being developed is biomass conversion, namely the conversion of cellulose based waste products to biofuels using an industrial process. There remain significant technical challenges to bring this technology to a commercial reality. Given the high cost of the transportation of the cellulose feedstock to the processing facility and high capital costs, this technology could be limited in scale to facilities that can produce 5-100 million gallons of biofuel annually. Therefore, there is and will remain a need for an industrial biofuels production technology that does not use or displace a feedstock that is for human/animal consumption, does not use arable land, can be made in very large quantities at a low price, and does not use precious fresh water resources. One such technology is the use of genetically enhanced photoautotrophic cyanobacteria, algae, and other photoautotrophic organisms to convert internal sugars directly to ethanol, butanol, pentanol and other higher alcohols and other biofuels.
For example, genetically modified cyanobacteria having constructs comprising DNA fragments encoding pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adh) enzymes are described in U.S. Pat. No. 6,699,696 (Woods, et al. for “Genetically modified cyanobacteria for the production of ethanol, the constructs and method thereof”). Cyanobacteria are photosynthetic bacteria which require light, inorganic elements, water, and a carbon source, generally carbon dioxide (CO2), to metabolize and grow. The process using technology described in Woods, et al. has enabled the development of the industrial production of ethanol on a massive scale using readily available, cheap feed substrates, namely water and CO2. The production of ethanol using genetically engineered cyanobacteria has also been described in PCT Published Patent Application WO 2007/084477 (Fu et al. for “Methods and Compositions for Ethanol Producing Cyanobacteria”).
The production of biofuels using genetically enhanced photoautotrophic cyanobacteria, algae, and other photoautotrophic organisms opens a new realm in the industrial production of biofuels. The primary benefit of this technology is the combining of the process of the conversion of solar energy into cellular biochemical energy (the production of internal cellular “sugars”) with the fermentation of these internal “sugars” directly into ethanol in one single cell. This “direct-to-ethanol” approach eliminates the need to separately grow and harvest the feed substrate then convert it to the biofuel. Other benefits of such a technology are the ability to use non-arable, non-productive marginal or desert land for production facilities, the ability to use saltwater, brackish water, fresh water or polluted water as a feed substrate, the ability to recycle enormous amounts of carbon dioxide into a transportation fuel and the ability to build massive scale production facilities with millions to billions of gallons of annual production capacity, all based on a genetically enhanced photoautotrophic organism.
Photoautotrophic organisms are those that can survive, grow and reproduce with energy derived entirely from the sun through the process of photosynthesis. Photosynthesis is essentially a carbon recycling process through which inorganic carbon dioxide (CO2) is combined with solar energy, other nutrients and cellular biochemical processes to synthesize carbohydrates and other compounds critical to life. Photosynthesis absorbs light in a limited range of the total solar spectrum, only in the wavelength range of 400-700 nm. This range only represents about half of the total solar energy. While at this time little can be done to expand the wavelength absorption range of photosynthesis, efforts can be made to optimize what energy can be absorbed.
In the open environment, the overall photosynthetic efficiency rarely exceeds 6%. A combination of factors including respiration during dark periods, the length of the photoperiod, the intensity and incidence of the light, the chlorophyll content, available nutrients and stress all further reduce the efficiency of open plants in the natural outdoor environment. In laboratory photobioreactors, it is possible to achieve a photosynthetic efficiency of greater than 24%. The goal of all photobioreactor production systems is to optimize the environmental conditions and fine tune the overall production process to achieve high biomass production and photosynthesis yields well beyond those capable in the natural environment and in open pond growing systems.
Previous efforts for larger scale production have focused on growing photoautotrophic organisms in open ponds or raceways that provide similar growing conditions found in nature. A major drawback of this approach is that growing conditions cannot be well controlled, resulting in uncertain production outputs, batch contaminations and uncertain manufacturing costs. These open systems are also not suited for efficiently cultivating the genetically enhanced (GE) organisms available today.
The current bottleneck for industrial photoautotrophic organism production is a lack of cost effective large-scale cultivation systems utilizing photobioreactors. Very high volumetric production is necessary to reduce the overall size of the installed production system as well as reduce the production and downstream processing cost. Key factors of such systems are a high biomass concentration per volume, high photosynthetic efficiency and the ability to have such systems use very little manmade energy. Designing cost effective, ultra large (millions to billions of gallons of annual production output) manufacturing systems that are needed to produce very large quantities of biofuels has been a major unsolved technical challenge to date.
Various studies have resulted in designs of closed photobioreactors for culturing photoautotrophic organisms utilizing various technologies. In these controlled environments, much higher biomass productivity was achieved, but the biomass growth rates were not high enough to offset the capital costs of the expensive systems utilized for the production of low cost biofuels. Research in this field has focused on developing photobioreactor systems of multiple designs including plate reactors (also known as flat panels), vertical gas-sparged photobioreactors, bubble column reactors, airlift reactors, external loop airlift reactors and tubular photobioreactors. Each of these systems allow for varying degrees of process control and optimization, resulting in improved growing conditions to achieve a predictable volume and cost. All of these systems have demonstrated the ability of higher volumetric biomass production when compared to open pond systems; however, all of these systems require significant external energy to operate the bioreactor systems. For biofuels production, it will be necessary to limit the amount of energy required to operate the system to ensure the greatest positive energy balance for the biofuels produced.
Photobioreactors are generally cylindrical or tubular in shape (pipe) (Yogev et al. in U.S. Pat. No. 5,958,761), are usually oriented horizontal, and require additional energy to provide mixing (e.g., pumps), thus adding significant capital and operational expense, they have no purposed airhead except that created by trapped O2. Oxygen, produced by photosynthesis can also become trapped in these types of systems and negatively inhibit growth and biofuel production. Photobioreactors, such as bubble columns or airlifts, may be oriented vertically and agitated pneumatically which can reduce the need for fluid pumping. These bioreactors are primarily or solely for biomass accumulation. Some photobioreactor designs rely on artificial lighting, e.g. fluorescent lamps, (such as described by Kodo et al. in U.S. Pat. No. 6,083,740). However, photobioreactors that do not utilize solar energy, but instead rely solely on artificial light sources, require so much energy input as to not be practical or cost effective for industrial scale production of biofuels. Fu and Dexter (WO 2007/084477) used GE 6×26 watt bulbs to provide light to the Bioflo R® 110 bioreactor system.
Several studies of algae cultured in photobioreactors have used narrow-bore tubes arranged in parallel and horizontal to the ground and on racks. These typically contain feed and harvest points to produce the biomass and require large surface areas. These systems rely on churning provided by pumping the biomass/growth medium through the piping at various high velocities. The cost of pumping in these systems will preclude them from being used in the production of biofuels on a large scale. Such systems are also not practical on a very large scale such as covering hundreds, if not thousands of hectares due to the high cost of the piping systems.
Bubble columns are typically translucent, large diameter vertically oriented containers filled with algae suspended in liquid medium, in which gases are bubbled in at the bottom of the container. Since precisely defined flow lines are not reproducibly formed in very large systems, it can be difficult to control the mixing properties of the system, which can lead to low mass transfer coefficients, poor photomodulation and low productivity.
Airlift reactors typically consist of vertically oriented concentric tubular containers, in which the gases are bubbled in at the bottom of the inner tube. The pressure gradient created at the bottom of the minor tube creates an annular liquid flow upward through the inner tube and then downward between the tubes. The external tube is made out of translucent material, while the inner tube is usually opaque. Therefore, the algae are exposed to light while passing between the tubes and to darkness while in the inner tube. The light-dark cycle is determined by the geometrical design of the reactor (height, tube diameters) and by operational parameters (e.g., gas flow rate).
Airlift bioreactors can have higher mass transfer coefficients and algal productivity when compared to conventional mechanically stirred systems. Analogous to mammalian cell production, large bubbles results in poor mass transfer of critical gases. Bubbles that are too small result in greater shear near the point of bubble creation and, therefore, more damaged or killed cells. Both damaged and killed cells can release components into the growth medium, that if too high, can greatly impact the health and thus the productivity of the system. However, control over the flow patterns within a very large airlift bioreactor to achieve a desired level of mixing and photomodulation is difficult or impractical. The energy requirement for an airlift photobioreactor is typically much lower than that for a stirred system and may be suitable for higher value products than commodity transportation fuel, but even the pumping costs required for an airlift photobioreactor are too great for low value commodity transportation fuels.
Moreover, because of geometric design constraints in most current systems, cylindrical-photobioreactors suffer from low productivity when used for large-scale outdoor algae production, due to factors related to light reflection and auto-shading effects (in which one column is shading the other). This technology is impractical for use in producing low value commodity transportation fuels such as ethanol.
It is important for optimum facility design and engineering to understand that when growing photosynthetic organisms at high density, shading of cells by other cells will reduce overall solar absorption.
Mixing mechanisms present a challenge in a bulk bioreactor and can be problematic once the cells pass from the mixing area of the bioreactor to the solar collection tubes where photosynthesis occurs. A major challenge to scale-up in a photobioreactor systems is increased shear stress from mixing or turbulence that results in cell damage (Gudin C., Dhaumont D. 1991. “Cell fragility is a key problem of microalgae mass production in closed photobioreactors,” Bioresource Technology 38:145-151). Cells are often more resistant to static hydrodynamic shear and less resistant to shear created by a liquid/air surface. Cell damage and lysis can occur at several points, including bubble creation, bubble rising and, as for mammalian cells, bubbles bursting at the liquid/air interface. Plant cell walls often contain cellulosic material that give them high tensile strength, but may have extremely low shear resistance. The fixed blade impellers or excessive airflows in airlift bioreactors produce high shear rates that result in cell breakage. The optimum level of turbulence for mixing, which creates shear stress for cells, is a result of fluid flow and gas velocity. As the cell density increases, the viscosity of the fluid rises, which works against uniform mixing and subsequent optimum mass transfer of nutrients. High airflow rates at high cell densities in an airlift bioreactor can result in shear becoming too great and cell breakage occurring. Algae, similar to other species of plants in suspension culture, vary in the resistance to shear. This has been a major challenge in developing a standard photobioreactor in which all cells can be grown.
Rapid alteration between high light intensities and darkness have consistently been shown to significantly enhance the efficiency of photosynthesis, with shorter cycles having greater effects (Matthijs, et. al. Application of light emitting diodes in bioreactors: flashing light effects and energy economy in algal culture. Biotechnol. Bioeng. 50:98-107). It has been speculated that the reduction of electron acceptors in photosystem II (PSII), with the corresponding oxidation of those acceptors in the dark, results in high solar energy capture during the light. A solar absorption tube system containing clear and dark areas with in-line mixers and an optimized residence time through fluid flow control should be capable of light to dark cycles from fractions of a minute to several minutes as dictated by the species of algae. A better way to get light dark transitions is to mix algae so they are alternately shaded by other algae or exposed to light. The overall efficiency of the system depends on its area and its output. A dark area is not contributing to output. The algae may be more efficient, but the system as a whole is not. But if shading by algae is used, light is always being absorbed by an active element. Janssen, M, et. al. (Scale-up aspects of photobioreactor s: effects of mixing induced light/dark cycles. J. Appl. Phycol. 12:225-237.) demonstrated that a light gradient, which occurs in natural sunlight over a typical day, has a significant impact on biomass yield from a given energy of light. It is expected that as one moves away from the equator, either north or south, that the light gradient will be more pronounced over the year, resulting in lower efficiencies when moving away from the equator. The design of many air-lift photobioreactors results in lower than expected biomass, even when carbon dioxide and other nutrients are increased. This is due to the physical design of most systems that facilitates medium length light/dark cycles, which have been shown to reduce biomass yield. Churning is too rapid in fast-moving airlift systems and bubble column systems.
Airlift systems can be designed to provide optimal mass transfer of oxygen and carbon dioxide, although at dry weight densities over 70 g/L maintaining adequate dissolved carbon dioxide becomes difficult. At high cell densities and during high rates of photosynthesis, the production of oxygen and the rapid utilization of carbon dioxide often required venting of the system. This becomes an issue in long sealed phototubes and is a primary reason that external loop phototubes have limited lengths when not vented. But these pipe or small diameter tube systems are predominantly for biomass production and ethanol as a directly produced product is not being made.
As early as 1959, Tulecke and Nickell, and in 1963 by Wang and Staba, produced 20 liter bioreactors for the culture of plant cells. In the early 1970's, Kato and his colleagues at the Japan Tobacco and Salt Public Corporation investigated the use of air in mixing up to 1500 liters. Later, a 20,000 liter system was used by Noguchi at the same corporation. In the mid-1980's, Wagner and Vogelmann demonstrated that the airlift system was superior to all others in providing good productivity and a well defined and controlled system of parameters, resulting in reproducible flow characteristics. They further suggested that fluid movement can be better controlled through the use of an internal draft tube through which the air mixture is bubbled. Although such systems can be scaled up significantly, their operation requires much external energy and they are therefore not cost effective for use as production systems for biofuels. In addition, mutual shading in vertical airlift systems has an impact on the total installed system capacity in a given footprint.
Javanmardian and Palsson (1991, High-density photoautotrophic algal cultures: design, construction, and operation of a novel photobioreactor system. Biotechnology & Bioengineering: 38, p 1182-1189,) developed an equation for the depth of light penetration. Applying this equation to a high density pond system suggests that at cell densities of 50 g/L, light penetration would be less than 2 mm. This demonstrates that light penetration clearly limits algal biomass production in typical open pond situations. Ogbanna and Tanaka (1997, Industrial-size photobioreactor s. Chemtech: 27(7), p 43-49.) demonstrated that photosynthesis is maintained with a light intensity of 7.3 μmol/m2/s. In addition, Lee and Palsson (1994, High-density algal photobioreactors using light-emitting Diodes. Biotechnology and Bioengineering: 44, p 1161-1167,) found that both light path and light intensity increased algal biomass production with light path possibly having a greater impact.
People have worked on ways to supplement natural light with artificial lighting in order to increase the efficiency of photosynthesis. Lee and Palsson (High density algal photobioreactors using light emitting diodes. Biotech. BioEng. Vol 44, 1161-1167:1994) used highly efficient light-emitting diodes (LED comprising gallium aluminum arsenide chips) to demonstrate that artificial light at specific wavelengths (680 nm monochromatic red light) could significantly increase the density of the cell culture. They found that supplementation with light at a wavelength of 680 nm would produce a cell concentration of more than 2×109 cells/ml (or more than 6.6% v/v) and an oxygen production rate as high as 10 mmol oxygen/L culture/h, using on-line ultrafiltration to periodically provide fresh medium. While this process is expensive and may be impractical for ultra large scales required in the production of biofuels, it did demonstrate the possibility of enhancing overall productivity through supplemental lighting.
Hu, et. al. (1998, Ultra-cell-density culture of marine green alga chlorococcum littorale in flat-plate photobioreactor. Applied Microbiology Biotechnology: 49, p 655-662) reported producing 84 g/L algae in a flat plate photobioreactor with a light intensity of 2,000 microeinsteins per second per meter squared. In a review of the literature up to that time 3-12 g/L were the more typical cell densities obtained in photobioreactors.
Fernandez, et. al. (2001, Airlift-driven external-loop tubular photobioreactors for outdoor production of microalgae: assessment of design and performance Chemical Engineering Science 56 (2001) 2721-2732) reported the results of the design of an airlift system which incorporated 80 m of clear tubing (pipe) as a solar receiver. The system was able to maintain high density algal cultures and used little external power to drive the system. This system's overall design and operation makes it an attractive candidate for large scale production, but the solar receiver system has limitations for massive scale production.
All current photobioreactor systems have various limitations for use in the massive scale industrial production of low cost biofuels. Most systems are not feasible because of their need for significant amounts of external energy for optimal operation. Another problem with existing systems is the high cost of the materials to build the systems. Many are mounted on expensive metal racking systems and are not placed on the ground. The most significant problem of all existing photobioreactors is that they are designed to maximize the production of biomass. The existing photobioreactors concentrate on the cells reproducing by division and increasing in number and mass with the goal of harvesting the algae or biomass and extracting products form the biomass in separate steps, or using the biomass itself, or drying the biomass. Current photobioreactor systems have various limitations for use in large scale industrial production of low cost biofuels using genetically enhanced photosynthetic microorganisms that make biofuels. Most systems are not feasible because of their need for significant amounts of external energy for optimal operation. Another problem with existing systems is the complexity and high cost of the materials to build the systems. Tube or pipe systems are traditionally made from acrylic or polycarbonate, and that is very expensive and not suited to large scale production. In additional the pipe or tube systems are placed on heavy metal racking to support the liquid culture off the ground, and that is also very expensive. Existing photobioreactors are designed to maximize the production of biomass. Existing photobioreactors concentrate on the optimization of biomass production through reproducing the cells by division and increasing in number and mass, or a particular lipid or protein with the goal of harvesting the microorganisms or biomass from the photobioreactor and then extracting the desired products from the biomass. Harvesting the product from the biomass requires significant energy and effort and typically requires that the microorganisms or biomass be replaced for the next cycle of biofuel production. Preferably, a photobioreactor system using genetically enhanced photoautotrophic organisms for the production of ethanol would collect the produced biofuel without having to remove the microorganisms or biomass from the photobioreactor. Traditional photobioreactors are unable to do this as they are designed to produce biomass and this would require the biomass to be harvested or separated from the water, saccrified and processed in many steps to get and end product of ethanol or lipid. In addition, cell division requires a significant amount of the cell's available biochemical energy. It is therefore desirable to maintain the culture in a steady state and convert as much cellular biochemical energy to ethanol as possible. Current bioreactor systems are not typically designed to maintain steady state cultures. Current industrial photobioreactor systems do not have a means for trapping ethanol produced by genetically enhanced photosynthetic microorganisms which is released directly into the culture medium. Therefore, in current bioreactor systems, the ethanol in the culture medium would be vented to the atmosphere while removing the saturated or supersaturated levels of O2 produced during photosynthesis volatilizing out of the culture medium.
To solve the problem of the inefficiencies involved in recovery of biofuels such as ethanol from harvested biomass in photobioreactors, the present invention overcomes the external energy usage and materials constraints of existing photobioreactor systems while maintaining the need for culture control and gas exchange necessary for maintaining high-density cultures for low cost biofuels production. Current systems have large requirements for energy to grow and maintain biomass. The present invention uses solar energy and only small amounts of manmade energy. The present invention provides for recovery of the biofuel from condensate in an upper portion of the photobioreactor and/or from a gas exhaust stream from an upper part of the chamber. The present invention uses a large airhead space above the culture medium as a means of allowing the ethanol to evaporate from the culture medium and enter the gas phase of the apparatus. The ethanol can then condense on the walls of the upper part of the apparatus and run into collection troughs, or the gas phase can be collected as it leaves apparatus and the ethanol enriched gases can go through external means of collecting the ethanol from the gas phase. As O2 is produced by the organisms in the culture, and any small trace amounts of excess CO2 introduced into the culture escape, and as water is changed from a liquid phase to a gas phase, O2 and ethanol gas, in addition to the ethanol in the condensate, will be pushed from the photobioreactor and the ethanol must be recovered in order to maximize the efficiency of the system. In the present invention this can be accomplished predominately with solar energy.
Furthermore, the present disclosure provides for recovery of the biofuel from condensate in an upper portion of the photobioreactor and/or from a gas exhaust stream from an upper part of the chamber. The apparatus and methods disclosed herein allow for at least approximately 80% to about 100% of the total biofuel product to be recovered by condensation from the upper portion of the photobioreactor and/or an exhaust gas stream with no recovery or at most only about 1% to about 20% of total biofuel production to be recovered from the biomass in a lower portion of the photobioreactor.
The production of biofuels using genetically enhanced photoautotrophic organisms on an industrial scale requires photobioreactor systems which cover hundreds, if not thousands, of hectares of land and contain millions, if not billions of gallons of water for organism growth. Such systems have never before been developed and pose significant engineering challenges. Because the product being produced is a low cost commodity, not a high value product, low manufacturing costs are critical for overall commercial success. This one constraint alone effectively eliminates all current designs of industrial sized photobioreactor systems for the manufacture of biofuels on a large scale. The current invention allows for photobioreactor systems which can efficiently produce biofuels from genetically enhanced photosynthetic microorganisms on a massive scale at low cost.
When exposed to sufficient light, such as sunlight, the genetically enhanced organisms release the biofuel into the aqueous growth medium where it evaporates as a gas into the upper part of the chamber. The water and biofuel condense on the inner surface of the upper part of the chamber and the droplets run down the internal surface into a collection trough, which in one embodiment uses gravity to drain to a lower area for distillation. Although this evaporation and condensation will happen continuously during the day, the greatest production of evaporation and condensation will most likely be at night during the time when there is a greater temperature differential between the inside of the bioreactor and the outside ambient air temperature.
Because the genetically enhanced organisms release the biofuel into the surrounding growth medium where it evaporates, none or very little of the culture has to be harvested or removed to recover the biofuel. This results in significantly increased efficiency and net energy gain from the system compared to photobioreactor systems that have to expend resources to remove most or all of the culture from the photobioreactor, separate the biomass from the culture, process the biomass by centrifugation or saccrification to extract a product, then new organisms have to be cultured and inoculated, and then the organism is replaced in the culture in the photobioreactor. None of these steps are trivial or inexpensive.
The photobioreactors and methods disclosed herein are especially applicable for use as large-scale outdoor photobioreactors, where sunlight is utilized as the light source. The term “large-scale” in reference to photobioreactors means photobioreactors having a volume greater than about 1,000 liters, or in some embodiments, greater than about 10,000 liters. The photobioreactors comprise closed shapes that can be any shape including but not limited to those with rectangular, triangular, cylindrical, circular, oval, irregular or polygonal cross-sections. The photobioreactors can be tube shaped, hexagonal or multisided domes, or circular domes. The photobioreactor is closed to the surrounding environment in the sense that loss by evaporation of biofuel is kept low and order to prevent: and to prevent contamination from heterotrophic bacteria and other organisms and their waste; evaporation of water; reduction in salinity changes; containment of the organisms; theft; and vandalism is minimized.
Biofuels able to be produced and released in the present invention include, but are not limited to, ethanol, butanol, pentanol and other higher alcohols. In a preferred embodiment, the biofuel is ethanol.
Constructs and methods for producing ethanol from genetically modified cyanobacteria have been disclosed (Ref: Woods et al., Fu/Dexter, Coleman et al.). These methods provide for the production and release of liquid ethanol into a culture medium from cyanobacteria exposed to sunlight and provided water, CO2 and nutrients. Methods and devices exist for the growth, maintenance, and harvest of algal or cyanobacterial biomass from aqueous cultures on a small scale. Most are designed for use in a laboratory and use artificial light to stimulate photosynthesis. Methods for separating ethanol from aqueous solutions are also disclosed. These separation devices all require energy in the form of manmade externally generated power to drive the separation process, whether it results from heating (distillation) or cooling (pervaporation). There are no devices currently available or described that provide for the large scale, outdoor growth and maintenance of cyanobacterial cultures producing ethanol that also enable the separation of ethanol from the aqueous culture in the same apparatus where the cyanobacteria are being cultured, wherein certain embodiments are made cheaply, are driven by solar power, use diurnal variation in light and temperature, and are designed for controlling the amount of nutrients, light, water, and CO2 to which the cyanobacteria are exposed.
The present invention solves these problems. It provides for an outdoor large volume closed photobioreactor for the continued daily production of ethanol from a culture media comprising genetically enhanced cyanobacteria or algae and separation of the ethanol from the culture media through evaporation by sunlight and subsequent condensation and ethanol collection in the same photobioreactor chamber in which the cyanobacteria grow. An embodiment of the invention includes the use of a single chamber composed of translucent plastic which contains a space for growing and maintaining the cyanobacteria in an aqueous culture medium, an airhead for evaporating the ethanol from the aqueous solution using sunlight, and a surface for condensing the ethanol from the evaporated gas. This embodiment has troughs for the collection of the condensed liquid ethanol that can then be moved through attached pipes to a separate apparatus for processing the ethanol to the desired purity. In another embodiment the upper part of the chamber of the photobioreactor has additional coolant compartments in thermal contact with the gas in the upper part of the chamber. A fluid coolant is passed through the coolant compartment that further cools the gas in the upper part of the chamber and enhances condensation of the biofuel into the collection troughs. The photobioreactor may be constructed as a single piece or as multiple pieces, such as a separate upper part of the chamber and lower part of the chamber, joined together, and in an embodiment is made from lightweight and inexpensive materials, including rigid materials such as extruded plastic, molded plastic domes, glass, fiber glass, plastic sheets or panels, and flexible materials, such as plastic film, or a combination of flexible and rigid materials. The upper part of the chamber is optionally coated with a material or constructed from materials that selectively filter out wavelengths of light. For example, the upper part of the chamber can be coated or constructed from a material that filters out potentially harmful UV light and/or only transmits a specified wavelength range optimal for photosynthesis by the organisms in the bioreactor.
The photobioreactor can contain ports for the injection of carbon dioxide or other gases. The injection of gas is designed to produce churning of the aqueous culture. Churning and mixing in the growth medium allows higher density cultures and higher biofuel production by minimizing the effects of mutual shading. Churning and mixing also provides for increased gas exchange from the growth medium to the gas phase in the upper part of the chamber and from the gas phase to the growth medium. Since oxygen is known to inhibit photosynthesis, removal of the oxygen produced during photosynthesis from the growth medium helps to optimize biofuel production. Churning also helps the carbon dioxide in the gas phase pass to the growth medium to support carbon fixation and increase biofuel production. Churning can be controlled through the use of baffles and dams, mixing devices, injection of gases such as carbon dioxide through the growth medium, as well as by the liquid flow through the photobioreactor. Excess oxygen in the growth medium or in the gas immediately above the culture can inhibit the cellular production of ethanol or other biofuels. Accordingly, ports or outlets can remove excess oxygen.
Thus, the current invention is a device that provides for the large scale, outdoor growth and maintenance of cyanobacterial cultures producing ethanol that also enable the separation of ethanol from the aqueous culture in the same apparatus where the cyanobacteria are being cultured, wherein certain embodiments are made cheaply, are driven by solar power, use diurnal variation in light and temperature, and are designed for controlling the amount of nutrients, light, water, and CO2 to which the cyanobacteria are exposed. One notes that the approach disclosed herein is not mentioned in the review article by C. U. Ugwu, et al., Photobioreactors for Mass Cultivation of Algae, 99 Bioresource Technology 4021 (2008).
The above discussion includes both information known to the art prior to the filing date and information forming part of the present inventive disclosure. Inclusion of any statement in this section, whether as a characterization of a published reference or in a discussion of technical problems and their solutions, is not to be taken as an admission that such statement is prior art.